The most common configuration for large trucks to transport goods on interstate highways is the tractor-semitrailer combination. The tractor is a power unit having a single steer axle at the front and tandem driving axles at the rear. The semitrailer is coupled to the tractor by a fifth wheel assembly attached to the tractor's frame. Operating conditions, such as the loads on the tractors, can vary greatly depending upon whether a semitrailer is fully loaded, lightly loaded, or even attached to the tractor at all. The tractors typically have suspension systems designed to provide desired ride and handling characteristics for different operating conditions.
The vast majority of modern highway tractors have pneumatic suspension systems that include air springs. The air springs are relatively light compared to other types of springs, and the stiffness of the air springs varies nearly in proportion to the load being carried. In addition, the natural frequency of a conventional air suspension varies little with changes in load, thereby allowing the suspension system to provide a soft ride under a wide range of loads. Air springs also permit the static height of the suspension to be maintained, independent of the load, through the use of a height control valve. The height control valve senses the position of the suspension and supplies or exhausts air from the air spring as required to maintain a constant ride height. These are particularly desirable features for large trucks since the load supported by the suspension system can change significantly between a fully loaded condition and a lightly loaded condition.
FIG. 1 schematically illustrates a side elevation view of a conventional truck 10 (i.e., a tractor) without the semitrailer attached to the fifth wheel assembly 22. The truck 10 has a frame 12 that supports a cab 14. A front steer axle 16 is coupled to a front end of the frame 12, and front and rear drive axles 18 and 20 are coupled to a rear end of the frame by a suspension system 21. The front drive axle 18 is typically forward of the fifth wheel assembly 22, and the rear drive axle 20 is rearward of the fifth wheel assembly.
During operation, the truck 10 is subjected to road inputs that excite various modes of vibration at different resonant frequencies. Two low frequency modes of vibration, referred to as "rigid-body" modes, correspond to the bounce and pitch motions of the frame on the suspensions. For conventional cars and trucks, the suspension bounce frequency is largely governed by the body mass of the vehicle, the distance between the axles, and the suspension's spring stiffness. Typical suspension bounce and pitch frequencies are approximately 1-3 Hertz (Hz) or cycles per second.
A third vibration mode having a higher natural frequency is referred to as "axle-hop," which is characterized by the out-of-phase motion of the axles 16, 18, and 20, with respect to the frame 12. As a result, the wheels and axles 16, 18, and 20 bounce up and down while the frame 12 has very little vertical movement. The axle-hop frequency is typically in the 10-12 Hz range for most modern cars and trucks, and is mostly a function of tire stiffness, suspension stiffness, and axle mass.
A fourth vibration mode is referred to as "frame beaming," which is characterized by structural vibration of the frame 12 in a vertical plane at the frame's natural frequency, known as the frame beaming frequency. The frame beaming frequency is typically in the range of 6-12 Hz, inclusive. If sufficient energy is transmitted through the suspension system 21 to the frame 12 at the frame beaming frequency, that energy will excite the frame, resulting in vertical movement of.the frame, thereby causing the truck's cab 14 to move up and down with the frame.
Unfortunately, the frame beaming frequency typically falls in the range where humans are most sensitive to vibration in the vertical direction. The frame beaming frequency also may coincide with the rotational frequency of truck tires at normal highway cruising speeds and, thus, frame beaming can be excited by tire and wheel non-uniformities such as imbalance, run-out, etc. Further, evenly spaced expansion joints on concrete highways or other non-uniformities in the road may also provide periodic excitation to the frame, very often near the frame beaming frequency.
As shown in FIG. 1 in phantom lines on the frame 12 and in an exaggerated amplitude, the vibration at the frame beaming frequency has nodal points where the frame experiences substantially no vertical motion during frame beaming. The forward most nodal point 30 generally occurs at a position near a rear engine mount on the frame. The rear nodal point 32 generally occurs at a position between the front and rear drive axles 18 and 20 and is generally aligned with the fifth wheel assembly 22.
Conventional air suspension systems are typically combined with shock absorbers or other dampeners that provide a portion of critical damping selected to provide the desirable balance of ride and handling qualities. Air suspension systems may also employ pneumatic damping in lieu of or in combination with shock absorbers.
One significant improvement in pneumatically damped vehicle suspension systems is described in U.S. Pat. No. 5,374,077, which is incorporated herein in its entirety by reference. The pneumatically damped vehicle suspension system provides damping which is load-dependent, so the amount of damping varies in approximate proportion to the load supported by the suspension system. Accordingly, a nearly constant fraction of critical damping is maintained over the normal range of operating loads.
The pneumatically damped suspension system utilizes two pneumatic damping circuits, including a high frequency damping circuit tuned to maximize damping at the axle hop frequency, and a low frequency damping circuit tuned to maximize damping at the suspension bounce frequency. Each high frequency damping circuit includes an air spring on one drive axle connected to an air spring on the other drive axle by a large diameter tube so air can substantially freely move between the air springs in response to a pressure differential between the air springs. At the higher axle hop frequency, the amplitude of the motion of the axles is large compared to that of the frame, and there is no intrinsic phase relationship between the two axles. If the two drive axles do not move in phase, air is transferred between air springs through the tube. The tube is sized to provide maximum damping via pumping losses at the axle hop frequency.
Each low frequency damping circuit includes an air spring pneumatically lumbed to a secondary air chamber by a small diameter tube. The air chambers and small diameter tubes are sized to provide maximum damping at the suspension bounce frequency. At the suspension bounce frequency, the frame and axles move in phase and the air springs deflect approximately the same amplitude, so very little air is transferred through the large diameter tubes between air springs. Thus, damping of suspension bounce is unaffected by the large diameter tubes between the air springs. Accordingly, the pneumatically damped suspension system is sufficiently tuned to provide damping at the axle hop and suspension bounce frequencies, thereby eliminating the need for hydraulic shock absorbers. While the pneumatically damped suspension system described in the U.S. Pat. No. 5,374,077 patent is very effective at providing damping at the axle hop and suspension bounce frequencies, the suspension system has no particular effect on frame beaming.